In typical low-pressure (LP) steam turbines used in power generation, steam leaving the last row of turbine blades flows through an annulus or exhaust flow passage between a bearing cone, surrounding an outer portion of the turbine shaft at the hub of the last-stage blades, and an exhaust flow guide extending from the cylinder structure surrounding the turbine blades and located adjacent the tip of the last-stage blades. The performance of a steam turbine may generally be improved by lowering the back pressure to which the last-stage blades of the turbine is subjected. Consequently, turbines often discharge to a condenser in which a sub-atmospheric pressure is maintained.
The exhaust steam discharging axially from the last-stage blades is typically directed to a condenser mounted below the turbine by turning the flow 90° from the axial to the vertically downward direction. The inner surface of the exhaust flow guide and the outer surface of the turbine bearing cone form a shape for the exhaust flow passage in which the steam passing from the last-stage turbine blades is preferably decelerated or, in other words, diffused. The diffusion causes a decrease in kinetic energy of the steam and a corresponding increase in pressure from the last-stage turbine blades to the exhaust flow passage exit. This exhaust flow passage exit pressure is influenced by the pressure in the condenser located after the exhaust hood. Since with diffusion there is an increase, in the flow direction, of steam pressure in the exhaust flow passage, there is a corresponding decrease in pressure of the steam at the exit of the last-stage turbine blades below the condenser pressure, called a negative hood loss, and a corresponding increase in turbine work output as compared to the work output which would occur in the absence of diffusion. Accordingly, an LP turbine provided with a diffuser can produce more power than if the diffuser was absent. However, for a fixed shape of the exhaust flow guide and the bearing cone, the performance of the passage is optimum at only one set of thermodynamic conditions.
The performance of the diffuser, as determined by a given shape of the exhaust flow guide and the bearing cone, and thus the performance of the LP turbine, is substantially affected by the pressure at the condenser. Further, the pressure at the condenser is largely a function of the ambient weather conditions, where the condenser pressure (also called back pressure) is typically higher in the summer months and lower in the winter months. For a given flow rate, the performance of the exhaust passage for a fixed-shape exhaust flow guide and bearing cone is optimum at only one value of back pressure. As the back pressure increases or decreases with seasonal changes, the performance of the exhaust passage becomes non-optimum. Non-optimum hood losses may be smaller negative values or the hood losses may rise into the positive range. For a positive hood loss, the exhaust passage no longer acts as a diffuser and the pressure at the exit of the last-stage blades is greater than the condenser pressure. It is often the case that a typical base-loaded steam turbine produces less power in the summer months than in the winter months. Accordingly, it is desirable to reduce hood losses in order to generate more power in the summer months. In order to accomplish this, it has been found desirable to make the shapes of the exhaust flow guide and/or the bearing cone variable or adjustable so that performance can be optimized for changing thermodynamic conditions.
U.S. Pat. No. 5,209,634 discloses an adjustable guide vane assembly in which adjustable vanes may be provided to change the cross-sectional area of an exhaust flow passage after the last row of turbine blades to control and minimize the pressure of the steam exiting the last row blades. It is noted that pivoted vane segments are disclosed located adjacent each other. A gap may form between edges of adjacent vanes of the vane segments which may permit flow of steam radially outwardly along the exhaust flow passage and potentially decrease the effectiveness of the exhaust flow passage.